Use of ceramics in the form of high temperature operating articles, such as components for power generating apparatus including automotive engines, gas turbines, etc., is attractive based on the light weight and strength at high temperatures of certain ceramics. One typical component is a gas turbine engine strut. However, monolithic ceramic structures, without reinforcement, are brittle. Without assistance from additional incorporated, reinforcing structures, such members may not meet reliability requirements for such strenuous use.
In an attempt to overcome that deficiency, certain fracture resistant ceramic matrix composites have been reported. These have incorporated fibers of various size and types, for example long fibers or filaments, short or chopped fibers, whiskers, etc. All of these types are referred to for simplicity herein as "fibers". The intent of including such fibers within the ceramic matrix was to make the matrix resistant to brittle fracture behavior. However, such fibers are not particularly effective in ceramic matrix composites where there is a strong bond between the fibers and the matrix. Such behavior may be attributed to the extremely high strain concentration existing at the tip of a brittle crack in the matrix, and the fact that the stress developed at such a strain concentration is high enough to fracture an individual reinforcing fiber; as each individual fiber is fractured, the load is transferred to the next fiber, and the fracture process is repeated sequentially until the crack has progressed across the entire section. In contrast, if the bond between the reinforcing fibers and the matrix is slightly weaker, the concentrated load at a crack tip causes localized separation of the fiber from the matrix, allowing the fiber to deform elastically over a somewhat greater portion of its length, simultaneously transferring a portion of the load to adjacent fibers.
Some fibers have been coated with certain materials which have been applied to prevent strong bonding between the reinforcement and matrix. However, some coatings are of carbon, or forms of carbon, or other material which will oxidize if exposed to air at an intended elevated operating temperature. Such exposure could occur if there would be microcracks in the matrix.
Oxidizing fibers, such as carbon, are potentially useful as reinforcement in ceramic composites, except that the system can become environmentally unstable in use; cracks in the ceramic matrix, even microcracks, can make the oxidizable fiber available to contact with oxygen in air at elevated operating temperatures experienced in the hot sections of power producing engines. Such oxidation of reinforcing fibers weakens or destroys the fiber structure or its function, leading to unacceptable weakening of the structural member.
Another problem relates to the fact that high sintering temperatures for ceramic particles about reinforcing fibers limit the kind of fibers which can be used. For example, many fibers deteriorate above about 1000.degree. C., well below required ceramic particle sintering temperatures.
There is another problem which may be encountered in the manufacture of ceramic matrix composite members. Rigid binders are commonly employed to maintain matrix particles and reinforcing fibers in their respective positions while the member is heated sufficiently to bond them together to form the member. Such binders do not permit the flow of matrix material to fill massive voids which might occur within the laid up structure of the member.